As stated previously, the invention relates to a vacuum sealing technology which has applications in nano- or micro-devices. It relates in particular to the fields of MEMS (Micro Electro Mechanical Systems), in particular RF micro-resonators for communication systems, requiring a high quality Q factor. Such a Q factor characterizes the frequency selectivity of the filters produced and is therefore directly associated with the performance of the micro-resonators. Under vacuum, friction is eliminated and the Q factor is considerably improved thereby. Moreover, the vacuum prevents any contamination of the micro-resonators, which could cause a slip of the resonance frequency.
The invention also relates to accelerometers, micromechanical DC-DC′ converters and field emission devices.
One particularly advantageous field of this application is that of uncooled infrared detectors. In this application, added to the need to seal a cavity under vacuum to improve the thermal insulation of the sensor, is the need to have a seal that is transparent to infrared radiation.
Several cavity sealing methods are known today, particularly standard packaging methods using individual metal or ceramic casings. Despite the possibilities of process automation, the cost of such packagings remain very high and sometimes prohibitive for certain applications.
Growing demand in the MEMS market has led to the development of less costly technologies in which the packaging is executed on the wafer itself, so as to enjoy the benefits of collective treatment.
According to this approach, two main families of methods can be distinguished: wafer bonding methods and thin layer vacuum deposition sealing methods.
In principle, wafer bonding methods consist of placing two wafers in contact and welding them by raising the temperature. These methods are complicated to apply, to execute the weld, and require a temperature rise that may be critical for certain devices and other active components.
The production of increasingly small devices now serves to carry out cavity sealing directly by thin layer deposition. These methods serve to lower the production cost because they only require the addition of a single step in the production process. The prior placement of the deposit under vacuum ensures the removal of the ambient atmosphere from the cavity to be sealed, and the completed deposit plugs the vents, thereby constituting a sealed barrier.
The deposition techniques used in the prior art are based on the principle of chemical vapour deposition (CVD, LPCVD, MOCVD, etc.).
According to these methods, a precursor gas of the deposit to be produced is injected at a pressure of typically between 10 and 103 Pa and decomposes on the substrate to be treated when raised to high temperature, typically >400° C., See, for example, U.S. Pat. No. 4,853,669.
This technique serves to obtain a good conformity of the deposit, that is, the deposit is deposited on the entirety of the part, regardless of the geometry thereof. For differences in level or vent clogging, this is a crucial advantage because the material is deposited on the root and on the sides of the hole to be filled.
However, the drawbacks of CVD and its derivative techniques, reside in the high temperature employed, which is necessary to achieve the decomposition of the precursors on the substrate. This high temperature precludes certain applications in which the thermal budget (i.e., amount) must remain as low as possible (micro-bolometers, applications on integrated circuits, etc.).
Furthermore, the working pressures, which determine the level of vacuum in the cavity, are several decades higher than those used in physical vapour deposition techniques (PVD).
Finally, risks of leaving organic waste in the cavity exist.
An alternative deposition technique is physical vapour deposition (PVD). According to this method, which takes place under partial vacuum, the vapour of the material to be deposited is created by a physical process, that is by heating or sputtering a target.
Contrary to CVD techniques, the vapour is condensed on the subject to be treated, without requiring additional heat input.
However, and in practice, where evaporation is concerned, the large majority of methods include a substrate heating step, in order to densify the deposits and improve the adhesion to the substrate.
Furthermore, it has been observed that PVD methods are not suitable for producing matching deposits, that is, capable of correctly matching the shapes of the substrate. This is because, whether by a process of evaporation or sputtering, the vapour generated has a spatial distribution that can be modelled mathematically by a cos(θ)n law, where θ is the angle between the atom emission direction and the axis of symmetry of the emitting source (the perpendicular to the target, in the case of a sputtering target), and n is an integer which determines the exact shape of the vapour cloud.
Hence the emission of the vapour is highly directional, thereby significantly affecting the good conformity of the deposit. Accordingly and so far, these techniques, derived from PVD have not been adopted for the described difference in level or hole plugging.
Some teams have nevertheless attempted such methods, but without success.
B. H. Stark and K. Najafi, Journal of Microelectromechanical Systems, Vol. 13, no. 2, April 2004 claim to have provided a low-temperature vacuum seal (T<250° C.). This is in fact a nickel deposit produced by electroplating, providing a seal cover. The final sealing of the hole of the cavity was attempted with several methods, including gold evaporation and sputtering techniques. In the case of PVD techniques, it was necessary to reduce the size of the hole to 0.15 μm because it proved impossible to plug larger (8 μm) holes. This is an important limitation for the preparation of the device, because reducing the size of the vent significantly lengthens the gas removal time.
This document therefore reveals that the plugging of vents by gold evaporation is fruitless. As an alternative strategy for the final sealing of the hole, the authors selected the welding of lead/tin alloy beads.
To try to avoid the directional emission of the vapour in PVD methods and therefore the non-conformity of the vent plugging, one solution for a person skilled in the art could reside in the heating of the substrate. In fact, although the species have a high directivity during the transport phase in the gas state, the thermal activation of the substrate to be treated serves to increase the mobility of the atoms on the surface and possibly to improve the covering capacity of the deposit. However, as already stated, the temperature rise of the substrate is highly unfavourable, and even incompatible with the applications considered by the present invention.
Another alternative which could be tested consists of the use of inclined or directional substrates, in order to compensate for the directivity of the vapour by an adjustment of the angle between the vapour and the device to be treated. However, this involves unconventional equipment, relatively unavailable in industrial circles, and costly.
A clear need therefore exists for a method for sealing by deposit not having the abovementioned drawbacks in comparison with the prior art methods, in particular which can be implemented at low pressure, at low temperature, without an organic or organometallic precursor, and in standardized deposition devices.